As the media for Direct Access Storage Devices (DASD) such as disk drives have realized higher track densities and decreasing track pitches, the actuators contained within the DASD have been similarly refined to accommodate the higher density recording. As the recording density has increased, the actuator capability for read/write resolution either has reached or is rapidly approaching its physical limit. The geometry and form factor of the disk drive limit the relative lever arm size, and further density increases are thereby limited unless alternative approaches are implemented.
The typical rotary actuator includes an arm, and a pivot therefore. Alternatively, linear actuators move the heads across the recording media in a linear movement. The actuator arm must be confined within the DASD and be movable with respect to the disk similarly located within the DASD. Any increase in the resonance frequency of the actuator arm which might serve to increase the track density of the DSAD device is severely limited by the physical size of the recording medium and the distance which must be covered by the actuator. Presently, the DASD disk surfaces and recordability are so improved as to permit an extremely high recording density.
Also, optical recording disks permit very high recording densities, allowing the extremely close placement of the recording tracks relative to each other on the disk of the DASD. With the disks having very fine resolution recording capability, the actuator and the actuator resonance frequency become the recording limiting factor. As track pitches decrease, there is a need for higher band width servo systems. Larger actuators typical of today's disc drives have resonances that are too low in frequency to allow adequate track following performance as track pitches decrease by a factor of 10 or more.
Maintaining the trend towards higher track densities and data rates in rotating memory devices require track following servo systems of increasing bandwidth for reliable storage and retrieval of data. The bandwidth requirements are even more severe in the emerging portable computing applications. Increase in the bandwidth is limited by the presence of mechanical resonance modes and nonlinearities in the voice coil motor (VCM) actuators. One approach to overcoming the problem is by using a dual or compound actuator. In such a mechanism, a VCM is used as a primary stage while the recording head is mounted on the secondary stage which in turn is mounted on the VCM. The secondary stage provides rapid small-motion and position correction to the recording head. Several dual-stage configurations have been proposed using electrostatic, piezoelectric and electromagnetic secondary actuators. The concern with conventional secondary stage configuration however, is additional reaction forces generated by the motion of the secondary actuator that are felt by the primary stage, could excite resonances in the primary stage, virtually eliminating the desired bandwidth improvements of the dual stage configuration. The use of a secondary actuator, smaller and lighter in size, attached to a larger gross positioning actuator, allows higher band width servos to be used. Prior art devices such as CD ROMS use such a system. It is desirable that forces used to actuate the secondary servo, not excite or interfere with the primary or gross motion structure. Such forces could excite resonances in the primary structure.
A dual stage or compound actuator necessarily consists of two masses. The smaller mass will contain the device to be positioned. The larger mass is typically the main body of the primary or gross movement actuator. Forces are applied between the smaller mass and larger mass by electromagnetic, electrostatic or other means. It is through these forces that resonances in the larger mass can be excited. Often the primary mass is made many times greater than the smaller mass in order to reduce the effect of these forces on the larger mass.
It is an object of the present invention to provide a secondary actuator design that eliminates the coupled forces between the secondary and the primary of a compound actuator design.